System and method for intelligent circuit breaking in adjustable speed drives

Abstract

An ASD circuit includes an input, solid-state switch rectifier bridge, DC link, and DC link capacitor bank. A pre-charge circuit is coupled between the input and the DC link capacitor bank and includes pre-charge relays operable in an on state that allows the AC power input to power the rectifier bridge during a normal operating state and an off state that allows the AC power input to pre-charge the DC link capacitor bank through a pre-charge resistor of the pre-charge circuit during a pre-charge operating state. A protection relay of a protection circuit is coupled between the pre-charge relays and the DC link capacitor bank, the protection relay operable in an on state that prevents the pre-charge circuit from connecting to the DC link capacitor bank when a capacitor short circuit occurs and an off state that allows the pre-charge circuit to electrically connect to the DC link capacitor bank.

What is claimed is: 
     
       1. An adjustable speed drive (ASD) circuit comprising:
 an input connectable to an AC source; 
 a solid-state switch rectifier bridge connected to the input to convert an AC power input to a DC power; 
 a DC link coupled to the solid-state rectifier bridge to receive the DC power therefrom; 
 a DC link capacitor bank comprising at least first and second capacitors connected to the DC link to smooth the DC link voltage, each capacitor of the DC link capacitor bank having a capacitor voltage thereacross; 
 a pre-charge circuit coupled between the input and the DC link capacitor bank, the pre-charge circuit comprising a plurality of pre-charge relays each operable in an on state that allows the AC power input to power the solid-state switch rectifier bridge during a normal operating state and an off state that allows the AC power input to pre-charge the DC link capacitor bank through a pre-charge resistor of the pre-charge circuit during a pre-charge operating state; and 
 a protection circuit comprising a protection relay coupled between the plurality of pre-charge relays and the DC link capacitor bank, the protection relay operable in an on state that prevents the pre-charge circuit from connecting to the DC link capacitor bank when a capacitor short circuit occurs and an off state that allows the pre-charge circuit to electrically connect to the DC link capacitor bank. 
 
     
     
       2. The ASD circuit of  claim 1  wherein, when the protection relay is in the on state and the plurality of pre-charge relays are in the off state, the protection and pre-charge relays prevent the DC link from receiving the DC power. 
     
     
       3. The ASD circuit of  claim 1  wherein the protection circuit further comprises:
 a voltage sampling circuit that measures a first capacitor voltage across the first capacitor; and 
 a logic circuit that receives the first capacitor voltage from the voltage sampling circuit, the logic circuit programmed to control the protection relay and the plurality of pre-charge relays based on the first capacitor voltage. 
 
     
     
       4. The ASD circuit of  claim 3  wherein the logic circuit is further programmed to:
 compare the first capacitor voltage to a first voltage threshold; and 
 if the at least one capacitor voltage crosses the first voltage threshold, operate the protection relay in the on state and the plurality of pre-charge relays in the off state to prevent the DC link from receiving the DC power. 
 
     
     
       5. The ASD circuit of  claim 4  wherein the logic circuit is further programmed to:
 compare the first capacitor voltage to a second voltage threshold; and 
 if the at least one capacitor voltage crosses the second voltage threshold, operate the protection relay in the on state and the plurality of pre-charge relays in the off state to prevent the DC link from receiving the DC power. 
 
     
     
       6. The ASD circuit of  claim 5  wherein the first voltage threshold comprises a capacitor overvoltage condition and the second voltage threshold comprises a capacitor undervoltage condition. 
     
     
       7. The ASD circuit of  claim 1  wherein the protection circuit further comprises:
 a voltage sampling circuit that measures a first capacitor voltage across the first capacitor and a second capacitor voltage across the second capacitor; and 
 a logic circuit that receives the first and second capacitor voltages from the voltage sampling circuit, the logic circuit programmed to control the protection relay and the plurality of pre-charge relays based on the first and second capacitor voltages. 
 
     
     
       8. The ASD circuit of  claim 7  wherein the logic circuit is further programmed to:
 compare the first capacitor voltage to the second capacitor voltage; 
 operate each of the protection relay in the on state and the plurality of pre-charge relays in the off state if a difference between the first capacitor voltage and the second capacitor. 
 
     
     
       9. A method of operating an adjustable speed drive (ASD) circuit that comprises a rectifier, a DC link having a DC link capacitor bank with two or more capacitors, a pre-charge circuit, a protection circuit, and an inverter, the method comprising:
 receiving an AC power at an input of the ASD circuit; 
 providing the AC power to the rectifier to convert the AC power to a DC power on the DC link; 
 measuring one or more capacitor voltages in the DC link capacitor bank via one or more voltage sensors; 
 providing the one or more measured capacitor voltages to a controller in operable communication with the one or more voltage sensors, the pre-charge circuit, and the protection circuit; 
 analyzing, via the controller, the one or more capacitor voltages during one of a pre-charge voltage condition and a normal voltage condition to detect if one or more capacitors of the DC link capacitor bank have developed a short circuit; and 
 selectively operating, via the controller, a protection relay in the protection circuit and a set of pre-charge relays in the pre-charge circuit in an on state and an off state, respectively, based on the analyzed one or more capacitor voltages, so as to prevent damage to the ASD circuit as a result of a short circuit on the DC link capacitor bank. 
 
     
     
       10. The method of  claim 9  wherein analyzing the one or more capacitor voltages comprises comparing a first measured capacitor voltage to a first voltage threshold; and
 if the first measured capacitor voltage passes the first voltage threshold, detecting a short circuit in the DC link capacitor bank. 
 
     
     
       11. The method of  claim 10  wherein analyzing the one or more capacitor voltages further comprises comparing the first measured capacitor voltage to a second voltage threshold; and
 if the first measured capacitor voltage passes the second voltage threshold, detecting a short circuit in the DC link capacitor bank. 
 
     
     
       12. The method of  claim 11  wherein comparing the first measured capacitor voltage to the first and second voltage thresholds comprises comparing the first measured capacitor voltage to a capacitor overvoltage threshold and an undervoltage threshold. 
     
     
       13. The method of  claim 9  wherein selectively operating the protection relay and the set of pre-charge relays based on the analyzed one or more capacitor voltages comprises simultaneously switching the set of pre-charge relays from the on state to the off state to disconnect the pre-charge relays from the rectifier and switching the protection relay from the off state to the on state to disable the set of pre-charge relays from electrically connecting to the DC link capacitor bank. 
     
     
       14. The method of  claim 13  further comprising disconnecting the AC power from the input of the ASD circuit based on the analyzed one or more capacitor voltages. 
     
     
       15. The method of  claim 9  wherein analyzing the one or more capacitor voltages comprises comparing a first measured capacitor voltage to a second measured capacitor voltage; and
 if the difference between the first measured capacitor voltage and the second measured capacitor voltage exceeds a voltage threshold, detecting a short circuit in the DC link capacitor bank. 
 
     
     
       16. An adjustable speed drive (ASD) circuit comprising:
 a rectifier comprising a silicon-controlled rectifier (SCR) bridge; 
 a DC link coupled to the rectifier to receive a DC link voltage therefrom, the DC link having a DC link capacitor bank thereon that includes at least two capacitors; 
 a pre-charge circuit comprising a set of pre-charge relays coupled to the DC link capacitor bank via the DC link, the pre-charge circuit configured to control an initial pre-charge of the DC link capacitor bank via the set of pre-charge relays; and 
 a protection circuit comprising:
 a protection relay operable in an on and an off position to selectively control a current flowing through the pre-charge circuit to the DC link capacitor bank; 
 a voltage sensing circuit for sensing a voltage across one or more of the at least two capacitors; and 
 a controller programmed to:
 receive an input comprising each voltage sensed by the voltage sensing circuit; 
 analyze the input using each sensed voltage to determine if a short circuit exists in the DC link capacitor bank; and 
 control the protection relay and the set of pre-charge relays based on the analysis. 
 
 
 
     
     
       17. The ASD circuit of  claim 16  wherein the controller is programmed to analyze the input by:
 comparing each sensed voltage to one or more voltage thresholds; and 
 if at least one sensed voltage crosses a respective one of the one or more voltage thresholds, controlling the protection relay and the set of pre-charge relays to disable the current from flowing to the DC link. 
 
     
     
       18. The ASD circuit of  claim 16  wherein the set of pre-charge relays of the pre-charge circuit are each operable in an on state that allows current to flow to the rectifier during a normal operating condition and an off state that allows current to pre-charge the DC link capacitor bank through a pre-charge resistor during a pre-charge operating condition;
 wherein the protection relay is coupled between the set of pre-charge relays and the DC link capacitor bank and is operable in an on state that prevents the pre-charge circuit from electrically coupling to the DC link capacitor bank and an off state that allows the pre-charge circuit to electrically couple to the DC link capacitor bank; and 
 wherein, when the protection relay is in the on state and the plurality of pre-charge relays are in the off state, the protection and pre-charge relays prevent the DC link from receiving the DC voltage. 
 
     
     
       19. The ASD circuit of  claim 16  wherein the voltage sensing circuit senses a first capacitor voltage and a second capacitor voltage; and
 wherein the controller is programmed to analyze the input by:
 comparing the first capacitor voltage to the second capacitor voltage; and 
 if a difference between the first capacitor voltage and the second capacitor voltage is greater than a voltage threshold, controlling the protection relay and the set of pre-charge relays to disable the current from flowing to the DC link. 
 
 
     
     
       20. The ASD circuit of  claim 16  wherein the controller is further programmed to:
 control the protection relay and the set of pre-charge relays to operate in:
 a pre-charge operating condition during which the DC capacitor bank is pre-charged and the rectifier and an inverter coupled to the DC link are disabled; and 
 a normal operating condition during which the rectifier is enabled to supply the DC link voltage to the DC link and the inverter is enabled to invert the DC link voltage to an AC output voltage; and 
 
 analyze the input using each sensed voltage to determine if a short circuit exists in the DC link capacitor bank during each of the pre-charge operating condition and the normal operating condition.